Redox flow batteries, also known as a flow cells or redox batteries or reversible fuel cells, are energy storage devices in which the positive and negative electrode reactants are soluble metal ions in liquid solution that are oxidized or reduced during the operation of the cell. Using two soluble redox couples, one at the positive electrode and one at the negative electrode, solid-state reactions are avoided. A redox flow cell typically has a power-generating assembly comprising at least an ionically transporting membrane separating the positive and negative electrode reactants (also called cathode slurry and anode slurry, respectively), and positive and negative current collectors (also called electrodes) which facilitate the transfer of electrons to the external circuit but do not participate in the redox reaction (i.e., the current collector materials themselves do not undergo Faradaic activity).
Differences in terminology for the components of a flow battery and those of conventional primary or secondary batteries are herein noted. The electrode-active solutions in a flow battery are typically referred to as electrolytes, and specifically as the cathode slurry and anode slurry, in contrast to the practice in lithium ion batteries where the electrolyte is solely the ion transport medium and does not undergo Faradaic activity. In a flow battery the non-electrochemically active components at which the redox reactions take place and electrons are transported to or from the external circuit are known as electrodes, whereas in a conventional primary or secondary battery they are known as current collectors.
While redox flow batteries have many attractive features, including the fact that they can be built to almost any value of total charge capacity by increasing the size of the cathode slurry and anode slurry reservoirs, one of their limitations is that their energy density, being in large part determined by the solubility of the metal ion redox couples in liquid solvents, is relatively low. The extent to which metal ion solubilities may be increased is limited.
In the field of aqueous electrolyte batteries, and specifically batteries that utilize zinc as an electroactive material, electrolytes that comprise a suspension of metal particles and in which the suspension is flowed past the membrane and current collector, have been described. See for example U.S. Pat. Nos. 4,126,733 and 5,368,952 and European Patent EP 0330290B1. The stated purpose of such electrodes is to prevent detrimental Zn metal dendrite formation, to prevent detrimental passivation of the electrodes, or to increase the amount of zincate that can be dissolved in the positive electrode as the cell discharges. However, the energy density of such aqueous batteries even when electrolytes with a suspension of particles are used remains relatively low. Such batteries cannot provide a high enough specific energy to permit practical operation of an electric vehicle, nor do they provide a substantial improvement in specific energy or energy density over conventional redox batteries for stationary energy storage, including for example applications in grid services or storage of intermittent renewable energy sources such as wind and solar power.